1. Field of the Invention
The present invention relates to a high performance semiconductor laser required for optical fiber communications, and to a manufacturing method therefor.
2. Description of the Prior Art
Research into single quantum well (SQW) and multiple quantum well (MQW) lasers, devices in which a quantum well structure is used as the active layer of the semiconductor laser, has been pursued in hopes of improving the characteristics of the semiconductor laser. Semiconductor lasers featuring a quantum well active layer can be expected to provide characteristics superior to those of common bulk active layer lasers because of the quantum size effect. For example, high efficiency, high output operation is possible at a low threshold level by increasing differential gain and reducing transverse magnetic wave (TM) emissions, and high speed response and low chirping can be obtained by increasing the relaxation oscillation frequency and reducing the line width amplification coefficient.
The use of a modulation doping MQW structure in which doping is applied to the quantum well barrier layer is also being studied in addition to the application of a strained quantum well and thin film barrier layers when differential gain is further increased. [K. Uomi, T. Mishima, N. Chinone, Appl. Phys. Lett., 51 (2), 13 July 1987 ]
A conventional semiconductor laser as shown in FIGS. 8 and 9 comprises an n-GaAs substrate 31, an n-GaAs buffer layer 32, an n-Ga.sub.0.6 Al.sub.0.4 As clad layer 33, an undoped AlGaAs wave guide layer 34, an undoped GaAs well layer 35, an undoped Ga.sub.0.8 Al.sub.0.2 As layer 36, Be-doped Ga.sub.0.8 Al.sub.0.2 As layer 37, modulation doping quantum well layer 38, p-AlGaAs clad layer 39, n-GaAs current constriction layer 40, SiO.sub.2 insulation layer 41, Zn diffusion region 42, an Au/Cr p-electrode 43, and an AuGeNi n-electrode 44.
The modulation doping quantum well layer 38 shown in FIG. 8 consists of the undoped GaAs well layer 35, and a barrier layer formed by the undoped GaAlAs barrier layer 36 and modulation doping (Be-doped GaAlAs) layer 37, as shown in FIG. 9(b).
In a conventional modulation doping quantum well semiconductor laser device as described, the current is input from the p-electrode 43, constricted by the Zn diffusion region 42, and then input to the modulation doping quantum well layer 38. The relaxation oscillation frequency, which is increased by increasing the differential gain, is confirmed to be increased in the modulation doping quantum well structure shown in FIG. 8, but not in an undoped quantum well structure.
As described above, the modulation doping quantum well layer 38 in a modulation doping quantum well semiconductor laser device such as that shown in FIG. 8 consists of the undoped GaAs well layer 35, and a barrier layer formed by the undoped GaAlAs barrier layer 36 and modulation doping (Bedoped GaAlAs) layer 37.
Comparing the relaxation oscillation frequency of this semiconductor laser with a uniformly doped quantum well semiconductor laser in which the well layer and barrier layer are uniformly doped as shown in FIG. 9 (d), an equivalent increase in the relaxation oscillation frequency is confirmed in both semiconductor laser constructions relative to an undoped quantum well semiconductor laser device. The probable cause for this is that post-doping heat treatment of the modulation doping constructions shown in FIGS. 9 (b) and (c) disperses the doping elements, effectively eliminating the modulation doping structure, and resulting in a uniformly doped construction. This is shown in FIG. 9 (a) . The modulation doping quantum well construction shown in FIG. 9 (b) has a high Be concentration only in the modulation doping layer before beat treatment, but a uniform Be concentration after heat treatment because of Be diffusion. As a result, there is no effective difference in the laser output of a uniform doping construction and a modulation doping construction. This is because the elements used for doping are diffused throughout the construction at high temperatures.